Hydrocarbons, and specifically C2+ hydrocarbons such as ethylene and aromatic hydrocarbons (e.g., benzene), can be typically used to produce a wide range of products, for example, polymers, plastics, resins, break-resistant containers, packaging materials, adhesives, rubbers, lubricants, dyes, detergents, drugs, explosives, pesticides, etc. Currently, for industrial scale applications, ethylene is produced by heating natural gas condensates and petroleum distillates, which include ethane and higher hydrocarbons, and the produced ethylene is separated from a product mixture by using gas separation processes. Benzene can be produced by a variety of processes, such as for example catalytic reforming, steam cracking, toluene hydrodealkylation, and toluene disproportionation.
Natural gas is an excellent clean energy resource, of which the primary component is methane (CH4). With an abundant supply of natural gas due to an explosive growth in shale gas and innovative drilling procedures (such as horizontal drilling and fracking), a lot of work has been directed towards direct conversion of methane into chemicals and fuels. The challenge of converting natural gas into transportable fuels and chemicals has been spurred by several emerging industrial trends, including rapidly rising demand for H2 (for upgrading lower-quality oils) and a global shortage of aromatic hydrocarbons caused by shifting refinery targets toward gasoline. The commercial processes that are currently in practice convert methane into CO and H2 and then convert the CO and H2 into desired chemicals or fuels in a number of different processes. Methane activation and its selective conversion to chemicals and fuels is difficult due to the strength of the C—H bond in methane and the subsequent higher reactivity of the resulting products such as ethane, ethylene, or methanol.
Conversion of CH4 into various chemicals has the potential of being more economical and environmentally friendly than other processes for producing the same chemicals, but is challenging because CH4 exhibits high C—H bond strength (434 kJ/mol), negligible electron affinity, large ionization energy, and low polarizability. CH4 can be converted to C2+ hydrocarbons in the presence of oxygen at high temperatures in a process known as oxidative coupling of methane (OCM). While the overall OCM is exothermic, catalysts are used to overcome the endothermic nature of the C—H bond breakage. Hundreds of catalytic materials have been synthesized and tested for OCM; however, the presence of O2 leads irreversibly to over-oxidation, resulting in a large amount of the thermodynamically stable end products CO2 and H2O, making the carbon utilization efficiency of OCM relatively low. When catalysts are used in the OCM, the exothermic reaction can lead to a large increase in catalyst bed temperature and uncontrolled heat excursions that can lead to catalyst deactivation and a further decrease in ethylene selectivity.
To achieve direct conversion of CH4 efficiently, the challenges lie in cleaving the first C—H bond while suppressing further catalytic dehydrogenation, avoiding both CO2 generation and coke deposition. CH4 can be converted by non-oxidative coupling of methane, direct pyrolysis of methane to acetylene, dehydroaromatization of methane to benzene, etc. However, coke formation continues to be an issue during methane conversion processes. Thus, there is an ongoing need for the development of CH4 conversion processes.